[Lens] Allow palette configuration for heatmap

[flash1293]

There is a nice palette configuration UI for table coloring, but heatmaps are statically using the temperature palette. We should expose the same palette configuration UI to allow the user to change the coloring scheme of the heatmap in a way they are accustomed to.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[dej611]

Created a first MVP for this: #102034

Note that the color continuity has been removed in the panel due to some limitations of the Heatmap visualization component and a chat with @markov00 from the dataviz team.

In the current PR the color continuity value set is Above and below range, which extends the color palette on both extremities as in the Heatmap defualt behaviour.
It would be possible to enable also the Above range strategy, possibly, but that requires some discussion about null values and out of range values representation:

The popover is showing an out of value there, colored in white (it depends on the current kibana theme). The previous cell is still white but it has no value, therefore no popup as well, and there's no way, other than hovering to know about the difference between the two.
In the datatable case this was "resolved" by the different representation of null values (-) and out of range values (actual value just not coloured), but in this case there's no way to differenciate it (yet).

As for the Below range and Within range strategies, there are also some component limitations which need to be addressed.

Probably before starting to address the technical limitations of the component it is worth discussing whether it makes sense for the Heatmap visualization to represent a filtered set of data via a filtered palette representation, or whether it makes more sense to handle the filtering at higher level, as Kibana/Lens filters directly.

[flash1293]

I think it makes sense to always do "Above and below range" for a heatmap - the color of a cell is whole deal, so it doesn't make sense to not assign a color. If they really want, the user could work around this by setting the first/last color stop to white (emulating out of range)

[flash1293]

cc @ghudgins @MichaelMarcialis